Flange Bolt Calculator: Preload, Gasket Sealing Stress and Torque

The preload in a set of flange bolts, the sealing stress it gives the gasket, and the torque to tighten them.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A bolted flange seals because the bolts squeeze the gasket. The squeezing force is the bolt preload, usually a high share of the bolt proof strength, such as 75 percent. Multiply the preload by the number of bolts for the total force, and divide by the gasket contact area for the sealing stress.

The gasket maker states a minimum seating stress. If the sealing stress is below it the gasket will not seat and the joint may leak. If it is far above the maximum the gasket can crush. The tightening torque is found from the preload, the diameter and the nut factor, and it is only a rough guide because friction varies.

Formulas used

  • F_i = \frac{\%}{100} S_p A_t
  • F_t = n F_i
  • \sigma_s = \frac{F_t}{A_g}
  • T = K F_i d

Worked example

With the default numbers in the calculator above:

  1. Tensile stress area of one bolt: 156.7 mm²
  2. Proof strength of the bolt class: 580 MPa
  3. Preload in one bolt: 68151 N (68.15 kN)
  4. Total bolt group force: 272603 N (272.6 kN)
  5. Gasket contact area: 13213 mm²
  6. Sealing stress on the gasket: 20.63 MPa
  7. Tightening torque for that preload: 163.6 N m

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Does it include the pressure load on the flange?

No. It gives the bolt preload only. The internal pressure acts to pull the flange apart and uses part of the preload, so the flange design must check that separately.

What nut factor should I use?

About 0.15 for lightly lubricated steel and about 0.2 for dry steel. The result is rough because friction varies a lot.

Where do I find the minimum seating stress?

On the gasket manufacturer data sheet.

Related calculators

Further reading: Hibbeler, Mechanics of Materials; Gere and Goodno, Mechanics of Materials; Shigley, Mechanical Engineering Design.

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